The present invention relates to flame sensors for use in conjunction with a boiler, furnace or similar combustion apparatus; and more particularly to such sensors which provide an indication presence and characteristics of a flame in a multiple burner system.
Large boilers and furnaces utilize several burners which produce a plurality of flames. An electronic control system for the burners often includes a mechanism for detecting the presence of the flame and for providing information about the flame characteristics. Such information is used in a control system to regulate safe operation of the burner. A flame scanner is incorporated in such control systems to detect the presence or absence of a burner flame in a single or multiple burner apparatus. When the burner is on and fuel is being ejected from the burner's throat, the scanner monitors the flame and produces a signal indicative of the condition, intensity and type of flame. It is therefore necessary for the flame scanner to be able to discriminate between flames from a burner to be scanned and the flames of adjacent burners and other background conditions.
Previous scanners utilized an optical sensor aimed at the flame to produce an electrical signal which was proportional in amplitude to the intensity of the light from the flame. The amplitude of the sensor signal, after band pass or high pass filtering, was relied upon to discriminate between on and off states of the burner flame. However, the magnitude of the signal is dependent upon a number of variables such as damper position, proximity of the flame to the sensor, type of fuel, and BTU content of the fuel. Similarly the other flames in a multiple burner system produce a widely varying background signal component in the sensor signal. A prominent problem with an amplitude dependent flame scanner is the varying magnitude of the sensor signal in the flame off and flame on states. As a consequence, the difference in sensor signal amplitude between the flame on and the flame off states often is too small in order to set reliable thresholds for discriminating between the flame states.
The failure of the scanner to be able to discriminate properly between the different flame states can result in the control system erroneously shutting down the entire burner or preventing the operator from starting the burner. In addition, an erroneous determination may occur due to the background signal component being interpreted incorrectly as indicating that the proximate burner being sensed is ignited. In such a situation, the proximate burner flame may be extinguished, but the flame scanner produces a signal to the control system indicating that the burner flame is on. This erroneous indication can result in the fuel valve remaining open allowing explosive fumes to accumulate in the burner chamber. Therefore, the control system must provide a mechanism for discriminating among signals produced by the burner flame to be sensed and those from other flames in a multiple burner system.